Ammonium nitrate high explosives are widely used either alone or in combination with other high explosives as binary explosives. They are particularly suitable for excavation and other applications requiring relatively low brisance number. Brisance number characterizes detonation rate, a factor which is reflected in the explosives' ability to move or displace large volumes of earth or similar material or to fracture hard rock.
The acceptance of ammonium nitrate explosives has been occasioned in part by its explosive characteristics, i.e., low brisance number, its low cost, ease of manufacture and susceptibility to mechanical handling. It is also a relatively powerful explosive, with some compositions having 120 percent the strength of TNT. The compound, alone, is not explosive unless retained under extremely high pressures and temperatures. For practical purposes it is necessary to add amounts of sensitizers sufficient to render the combination explosive. A wide variety of sensitizers are known for this purpose, including natural and synthetic organic carbonaceous materials such as mineral and vegetable oils, waxes, animal fats, distillate residues, sugars, coal, the several forms of carbon including coke, graphite, etc., elemental sulfur and powdered, flaked or otherwise finely divided metals such as aluminum, zinc and magnesium.
Suitable combinations of these sensitizers with ammonium nitrate can be produced by numerous procedures. Adequate combinations have been formed by simply mixing ammonium nitrate granules or powder with fuel oils or by adding the sensitizer to a melt or solution of ammonium nitrate and then solidifying and granulating the combination.
All of these approaches suffer from one or more of several disadvantages. The simple admixture of oils and ammonium nitrate particles forms a sticky mass in which all the particles adhere to each other making them difficult to handle. Ammonium nitrate solutions or melts are incompatible with many of the more preferred sensitizers such as hydrophobic mineral oils, and their use requires additional solidification and granulation procedures described above. Many procedures require melted or dissolved ammonium nitrate and, accordingly, do not take advantage of the fact that ammonium nitrate is widely available in particulate or prill form.
It would be preferable to convert these particles directly to useable explosives without the need of destroying and then reproducing the original physical form. Smaller particles afford ease in mechanical handling in transportation and container loading.
However, an appropriate manufacturing method must provide even sensitizer distribution, should not require the use of types or amounts of inert materials that detract substantially from product quality, or result in the formation of cohesive particles.
Presumably these objectives could be accomplished by mixing the sensitizer and other components with the ammonium nitrate melt or solution during the prilling operation itself. This is often impossible, however, due to incompatibility of the sensitizer with melts or solutions. This approach also complicates prilling operations in a manner which may or may not be easily solvable or economically acceptable. It is hardly feasible to convert commercial prilling facilities into piecemeal explosive manufacturing operations.
Due to the wide availability of particles or prills as such, it would of course be desirable to devise a method of combining sensitizers with available particles on a limited scale without appreciable difficulty, a method that would not require interference with large-scale particle production in the first instance. This approach is also complicated, however, by several factors, primarily physical structure (low porosity and permeability) and sensitizer incompatibility. Ammonium nitrate prills have extremely low porosity, certainly insufficient surface porosity to retain the necessary amounts of sensitizer without agglomeration. Thus, very little if any material can be added to the particle surface by a simple procedure.
It is therefore one object of this invention to provide an improved particulate ammonium nitrate explosive composition. Another object is a provision of a simple method for combining ammonium nitrate particles and sensitizers. Another object is the provision of a method for converting commercially available ammonium nitrate particles or prills into non-adherent, free-flowing, particulate explosives.